1. Technical Field
The present invention relates generally to cellular wireless communication systems; and more particularly to the processing of data communications received by a wireless receiver in such a cellular wireless communication system.
2. Related Art
Cellular wireless communication systems support wireless communication services in many populated areas of the world. While cellular wireless communication systems were initially constructed to service voice communications, they are now called upon to support data communications as well. The demand for data communication services has exploded with the acceptance and widespread use of the Internet. While data communications have historically been serviced via wired connections, cellular wireless users now demand that their wireless units also support data communications. Many wireless subscribers now expect to be able to “surf” the Internet, access their email, and perform other data communication activities using their cellular phones, wireless personal data assistants, wirelessly linked notebook computers, and/or other wireless devices. The demand for wireless communication system data communications will only increase with time. Thus, cellular wireless communication systems are currently being created/modified to service these burgeoning data communication demands.
Cellular wireless networks include a “network infrastructure” that wirelessly communicates with wireless terminals within a respective service coverage area. The network infrastructure typically includes a plurality of base stations dispersed throughout the service coverage area, each of which supports wireless communications within a respective cell (or set of sectors). The base stations couple to base station controllers (BSCs), with each BSC serving a plurality of base stations. Each BSC couples to a mobile switching center (MSC). Each BSC also typically directly or indirectly couples to the Internet.
In operation, each base station communicates with a plurality of wireless terminals operating in its cell/sectors. A BSC coupled to the base station routes voice communications between the MSC and the serving base station. The MSC routes the voice communication to another MSC or to the PSTN. BSCs route data communications between a servicing base station and a packet data network that may include or couple to the Internet. Transmissions from base stations to wireless terminals are referred to as “forward link” transmissions while transmissions from wireless terminals to base stations are referred to as “reverse link” transmissions. The volume of data transmitted on the forward link typically exceeds the volume of data transmitted on the reverse link. Such is the case because data users typically issue commands to request data from data sources, e.g., web servers, and the web servers provide the data to the wireless terminals.
Wireless links between base stations and their serviced wireless terminals typically operate according to one (or more) of a plurality of operating standards. These operating standards define the manner in which the wireless link may be allocated, setup, serviced and torn down. One popular cellular standard is the Global System for Mobile telecommunications (GSM) standard. The GSM standard, or simply GSM, is predominant in Europe and is in use around the globe. While GSM originally serviced only voice communications, it has been modified to also service data communications. GSM General Packet Radio Service (GPRS) operations and the Enhanced Data rates for GSM (or Global) Evolution (EDGE) operations coexist with GSM by sharing the channel bandwidth, slot structure, and slot timing of the GSM standard. The GPRS operations and the EDGE operations may also serve as migration paths for other standards as well, e.g., IS-136 and Pacific Digital Cellular (PDC).
In order for EDGE to provide increased data rates within a 200 KHz GSM channel, it employs a higher order modulation, 8-PSK (octal phase shift keying), in addition to GSM's standard Gaussian Minimum Shift Keying (GMSK) modulation. EDGE allows for nine different (autonomously and rapidly selectable) air interface formats, known as Modulation and Coding schemes (MCSs), with varying degrees of error control protection. Low MCS modes, (MCS 1-4) use GMSK (low data rate) while high MCS modes (MCS 5-9) use 8-PSK (high data rate) modulation for over the air transmissions, depending upon the instantaneous demands of the application and the operating conditions.
A typical wireless device, e.g., hand held wireless device, that supports these operations typically includes at least a baseband processor and a Radio Frequency (RF) front end among other components. The baseband processor and RF front end often times reside on differing integrated circuits. The baseband processor provides a digital interface to other components and also provides an analog interface to the RF front end. For transmit operations, the baseband processor receives digital information for transmission and creates a baseband (or low Intermediate Frequency (IF)) signal that carries the digital information. The RF front end receives the baseband signal, up converts the baseband signal to an RF signal, and provides the RF signal to an antenna for transmission. For receive operations, the RF front end receives an RF signal from the antenna and down converts the RF signal to a baseband (or low IF) signal. The baseband processor receives the baseband (or low IF) signal and extracts digital information from the baseband signal.
In order to support high data rate operations, e.g., GPRS, EDGE, etc., the RF front end and baseband processor must meet strict operating conditions. One such operating condition requires that the RF front end and baseband processor correctly extract digital information from relatively weak received RF signals. In meeting this condition, the baseband signal produced by the RF front end to the baseband processor must be of high quality, which requires that the RF front end perform its amplification, filtering, and down conversion operations without degrading the signal. Most RF front ends, however, introduce significant DC offset into the baseband signal. Some RF front ends may produce a baseband signal having more DC offset than a weak information signal contained therewith.
Typical Super heterodyne RF front ends generate residual DC offsets of 100s of millivolts in baseband signals that they produce. Typical Direct Conversion RF front ends generate residual DC offsets of 100's of millivolts in baseband signals that they produce. RF front ends that produce a Very Low IF (VLIF) signal typically produce less DC offset than do Super heterodyne RF front ends. However, VLIF RF front ends typically place very high attenuation at the VLIF, e.g., 100 KHz, to suppress DC components prior to de-rotation, which disrupts demodulation operations. Because residual DC offset produced by an RF front end is typically independent between frames, removal by a servicing baseband processor is difficult. In servicing higher order modulations, final uncorrected DC offset residuals must be no greater than −20 dB relative to the signal for a GMSK burst and preferably no greater than −35 dB relative to the signal for 8-PSK. Thus a need exists for a wireless receiver that meets these stringent DC offset performance criteria.